1. Field of the Invention
The present invention relates to a bridge rectifier circuit used in a synchronous generator or a synchronous motor-generator, having an armature and a field winding, that is mostly mounted on a vehicle and serves as a starter-motor when starting the combustion engine as well as a generator after the engine start.
2. Description of the Related Art
In a synchronous generator or a synchronous motor-generator that has an armature and a field winding and serves as a starter-motor when starting the combustion engine as well as a generator after the engine start, to date, technologies have been disclosed in, for example, Japanese Patent Publication No. 2959640 (hereinafter referred to as Patent Document 1), in which power loss during generating and charging operations can be reduced using, as rectifier elements for the rectifier circuit, metal-oxide semiconductor field-effect transistors (hereinafter referred to as MOSFET or FET). The technology disclosed in Patent Document 1 is as follows: when a voltage is applied between the drain and the source of an FET in such a way that voltage at the source is higher than that at the drain (when a reverse voltage is applied therebetween), a voltage is applied between the gate and the source thereof so that the FET is turned on, thereby reducing power loss.
However, in such a conventional technology as disclosed in Patent Document 1, when a voltage generated by the armature winding is outputted through the bridge rectifier circuit to a battery for its charging, or a current thereby is supplied to the battery, determination as to whether voltage at the source of the FET is higher than that at the drain thereof (that is, whether a reverse voltage is applied therebetween) is made based on a difference in voltage across the two terminals of the FET. Therefore, in determining the voltage difference, a determination threshold for the reverse voltage needs to be set within a range of a forward voltage drop (around 1V to 1.5V) of a parasitic diode formed in the opposite direction of the drain to the source so that the FET is not erroneously turned on, taking also into account temperature dependence in which voltage drop between the drain and the source varies with temperature. Therefore, the range of the determination voltage must be severely limited, resulting in deterioration of detectability, which has in turn caused a problem in that an operation area in which power loss in the bridge rectifier circuit can be reduced by taking control of turning on the FET must also be limited.
Moreover, there has been another problem in that protections and the like against abnormal high-current failures due to short-circuit of FETs connected in series in the bridge rectifier circuit or abnormal currents when the motor-generator is operating as the motor at the engine start are also restricted.
Furthermore, in a case of using a method in which, when voltage at the source is higher than that at the drain (when a reverse voltage is applied therebetween), a voltage is applied between the gate and the source so that the FET is turned on to reduce power loss, if a failure in which voltage at the source becomes higher than that at the drain occurs due to a power-source short-circuit failure causing the battery voltage to be applied to FET output terminals or due to other abnormalities and failures, the FET is controlled to turn on despite the presence/absence of a voltage generated by the armature winding, which resultantly raises a fear in that failures can not be properly detected so that an overcurrent might flow through the bridge rectifier circuit, getting the circuit into a serious failure state such as burnout.